In the low-pressure stage of the blade cascade in a steam turbine, performance degradation (moist loss) due to a drain (water drops) produced in moist steam and erosion due to a drain attack, which is a collision of the drain on a portion of a turbine, are seen as problems. Thus, there has been developed a steam turbine including a mechanism for discharging a drain in moist steam from a steam passage of a turbine. For instance, Japanese Unexamined Patent Application No. 7-42506 discloses a configuration in which a slit is disposed along the circumferential direction on an outer race that holds stator blades so that a drain in steam is discharged through the slit to the outside of a steam passage.
Here, in a downward exhaust turbine including a condenser disposed below a low-pressure chamber, steam having exited from the final-stage rotor blades of the low-pressure chamber is guided by a flow guide to flow downward, and then drawn into the condenser. Thus, when a drain is collected from a steam passage by a slit described in Japanese Unexamined Patent Application No. 7-42506, for instance, it is only required to provide a through hole for discharging drains on a blade base attached on a low-pressure turbine casing to cause the drain to be introduced into the condenser by the pressure difference between the outlet and inlet of the through hole.
Meanwhile, another known steam turbine of a condensing type is an axial-flow exhaust turbine that discharges steam having passed through a blade cascade in a turbine axial direction. The axial-flow exhaust turbine can restrict exhaust loss (pressure loss due to exhaust gas) low, which makes it possible to maintain high energy efficiency. The axial-flow exhaust turbine is also advantageous in terms of the layout because it is not necessary to dispose a condenser at a lower part of the turbine. In a common axial-flow exhaust turbine, an exhaust chamber is disposed on the outlet side of the blade cascade, i.e., on the downstream side in the turbine axial direction, of a casing. The casing surrounds the blade cascade in which a plurality of rotor blades and stator blades are arranged in rows. Normally, a condenser is disposed on the further downstream side of the exhaust chamber in the axial direction, communicating with the exhaust chamber.
In the above axial-flow exhaust turbine, the condenser is arranged adjacent to the exhaust chamber in the axial direction. Thus, discharging a drain from a steam passage may be a problem.
In regard to the above issue, Japanese Unexamined Patent Application No. 10-18807 discloses a configuration of a drain-discharging device employed in an axial-flow exhaust turbine. The drain-discharging device includes a drain hole disposed on a blade base on which the final-stage stator blades are supported, and a pocket communicating with the drain hole. The pocket and an exhaust chamber are in communication with each other through a plurality of piping. With this device, a drain in a steam passage is drawn in by the negative pressure in the exhaust chamber connected to the condenser to be introduced into the exhaust chamber through the drain hole, the pocket, and the plurality of piping. Then, the drain reaches the condenser with the exhaust gas.